Coomber Rotary Engine

First learned of this delightful engine while attending the PRIME show in Oregon. A most prolific modeler, Marlyn Hadley, had one on display.

Its have not illustrated the valve linkage, as not exactly sure what it looks like, It appears to be a rotary type, incorporated into the main drive shaft. Steam would be admitted to one end of the cylinder at a time, just as in any other double-acting steam engine.

The inner dimension of the stationary ring is not circular, but is slightly elliptical. The main bearing is offset from the center of this ellipse by a one half the stroke length.

CO2 Motor

This style engine could be powered by steam (I've heard of at least one) but is more commonly seen in small model airplane engines powered by compressed air or CO2 (carbon dioxide) gas. The popular Air Hogs toy airplanes are propelled by this style motor.

At the top of the stroke, the pin on the cylinder presses the ball valve upward, admitting high pressure gas into the cylinder.

The gas expands, driving the piston downward.

when the piston advances past the exhaust port, the high-pressure gas is released.

Flywheel (or propeller) momentum carries the piston upward to complete the cycle.

This animation also illustrates the CO2 reservoir, or "fuel tank." Compressed CO2 is a liquid and becomes a gas as the pressure is released. Another way to state this is that the liquid CO2 boils at normal atmospheric temperature and pressure, so one might say this engine runs on "CO2 steam."